This invention relates generally to semiconductor process equipment, and more particularly, to a method and systems for controlling gas flow to a semiconductor processing reactor.
Semiconductor processing typically involves the formation of one or more layers on a semiconductor substrate. For example, silicon epitaxy, sometimes called epi, is a process in which one or more layers of single-crystal (monocrystalline) silicon are deposited on a monocrystalline silicon wafer.
FIG. 1 is a schematic representation of a semiconductor processing system 10 in accordance with the prior art. As shown in FIG. 1, system 10 included a susceptor 12 enclosed within a barrel reactor 14. Susceptor 12 supported a plurality of substrates 16, typically monocrystalline silicon wafers.
During processing, substrates 16 were heated with an external radiation source such as tungsten halogen lamps, resistive heating elements and/or RF heaters (not shown).
A process gas was introduced into reactor 14 through one or more injector ports 18. The process gas typically included trichlorosilane although other process gases besides trichlorosilane sometimes were used depending upon the particular type of layer that was deposited. The process gas reacted with heated substrates 16 resulting in the deposition of layers on substrates 16 as those skilled in the art understand. The spent process gas was exhausted through a vacuum pump 20 to exhaust 23. Alternatively, the spent process gas was directly exhausted to exhaust 23 and vacuum pump 20 was not used.
Of importance, to insure the consistency and quality of the deposited layers on substrates 16, the composition and mass flow rate of the process gas delivered to reactor 14 was carefully controlled. For this reason, system 10 included a gas flow control system 21 coupled to injector ports 18 by a process gas line 24. Gas flow control system 21 was generally located in a gas cabinet 22 located at a distance from reactor 14.
Located within a gas bottle cabinet 49 were three process gas sources 26, 28, 30 and a carrier gas source 50. Illustratively, process gas sources 26, 28, 30 and carrier gas source 50 included compressed gas cylinders containing process gases A, B, C, and carrier gas CG, respectively.
Process gas sources 26, 28, 30 were coupled to a gas manifold 38 of system 21 through mass flow controllers (MFCs) 32, 34, 36, respectively, of system 21. Gas manifold 38 had a plurality of input ports 38A, 38B, 38C, a first output port 38Y and a second output port 38Z. MFCs 32, 34, 36 controlled and regulated the mass flow rates of flows of process gases A, B, C from process gas sources 26, 28, 30, respectively, to input ports 38A, 38B and 38C, respectively, of gas manifold 38. Output port 38Y of gas manifold 38 was coupled to process gas line 24 by valve 40 of system 21. Output port 38Z of gas manifold 38 was coupled to an inlet of vacuum pump 20 (generally referred to as exhaust 23) by valve 42 of system 21. An outlet of vacuum pump 20 was coupled to exhaust 23. Alternatively, vacuum pump 20 was not used and output port 38Z of gas manifold 38 was directly coupled to exhaust 23 by valve 42.
Carrier gas source 50 was coupled to process gas line 24 through a mass flower controller (MFC) 52 of system 21. MFC 52 controlled and regulated the mass flow rate of a flow of carrier gas CG from carrier gas source 50 to process gas line 24.
To illustrate the operation of gas flow control system 21, assume that a heavily doped P type silicon layer was to be deposited after which a lightly doped P type silicon layer was to be deposited on substrates 16. In this example, process gas C was a P type dopant gas. Further, process gas B was a source of silicon, e.g., was trichlorosilane.
Initially, to form the heavily doped P type silicon layer, valve 42 was open and valve 40 was closed. Process gases B. C from process gas sources 28, 30 flowed through MFCs 34, 36, respectively, to gas manifold 38. In gas manifold 38, process gases B, C mixed (the mixture of process gases B, C is hereinafter referred to as high dopant concentration process gas). The high dopant concentration process gas flowed from gas manifold 38 through valve 42 to exhaust 23.
As those skilled in the art understand, gas must flow through a mass flow controller (MFC) for a certain period of time after activation of the MFC to allow the mass flow rate of the flow of gas through the MFC to stabilize and to allow the MFC to accurately control the mass flow rate of the flow of gas. Thus, the flow of the high dopant concentration process gas to exhaust 23 continued until the mass flow rates of the flows through MFCs 34, 36 stabilized. Valve 40 was opened and valve 42 was closed thereby providing the high dopant concentration process gas through process gas line 24 and injector ports 18 into reactor 14. The high dopant concentration process gas reacted with heated substrates 16 and formed the heavily doped P type silicon layer on each of substrates 16.
After a predefined time period, valve 40 was closed to stop the flow of the high dopant concentration process gas into reactor 14 and to stop the deposition of the heavily doped P type silicon layer on substrates 16. FIG. 2 is a graph of the concentration of the high dopant concentration process gas in reactor 14 verses time after shutting-off the flow of the high dopant concentration process gas to reactor 14 by closing valve 40.
Referring to FIGS. 1 and 2 together, time T=0 is at the end of the predefined period when valve 40 was closed. After valve 40 was closed, the concentration of the high dopant concentration process gas gradually decreased in reactor 14 as the high dopant concentration process gas was displaced by carrier gas CG supplied from carrier gas source 50. In particular, a length of time T=T1, e.g., thirty seconds to two minutes or more, after valve 40 was closed passed before the high dopant concentration process gas was fully removed from reactor 14. Undesirably, the high dopant concentration process gas continued to react and formed a transition layer on the newly formed heavily doped P type silicon layer until the high dopant concentration process gas was fully removed from reactor 14.
After the high dopant concentration process gas was fully removed from reactor 14, the lightly doped P type silicon layer was deposited. Valve 42 was opened and process gas A, hereinafter referred to as low dopant concentration process gas, flowed through MFC 32 through valve 42 to exhaust 23 until the mass flow rate of the flow through MFC 32 stabilized. Valve 40 was opened and valve 42 was closed thereby providing the low dopant concentration process gas into reactor 14. The low dopant concentration process gas reacted with heated substrates 16 and formed the lightly doped P type silicon layer on substrates 16.
FIG. 3 is a graph of dopant concentration versus depth in a substrate 16 in accordance with the prior art process described above. Referring to FIG. 3, the top of the heavily doped P type silicon layer described above (hereinafter referred to as HD layer L1) was located at a distance D1 from a surface of substrate 16.
Referring to FIGS. 1 and 3 together, after HD layer L1 was formed with a desired thickness D1, valve 40 was closed to stop the flow of the high dopant concentration process gas to reactor 14. However, after closing of valve 40, transition layer TL was formed on HD layer L1.
Since the concentration of the high dopant concentration process gas diminished in reactor 14 after valve 40 was closed, the dopant concentration of transition layer TL gradually changed from heavily doped HD at the bottom of transition layer TL to lightly doped LD at the top of transition layer TL. The lightly doped P type silicon layer (hereinafter LD layer L2) was formed on transition layer TL.
As the art moves towards smaller high speed devices, it is important that the transition between layers be abrupt. In particular, referring to FIG. 3, it is important to reduce or eliminate transition layer TL between the top of HD layer L1 and the bottom of LD layer L2. Unfortunately, it is not possible to instantaneously purge the reaction chamber. Thus, the current generation of semiconductor processing reactors do not appear suitable for manufacturing the substrates used in making the smaller high speed devices.
In addition to having abrupt transitions between layers, it is also important to accurately control the dopant concentration within a layer in any particular batch and also from batch to batch. To illustrate, referring to FIG. 3, if the P type dopant concentration of the high dopant concentration process gas which formed HD layer L1 was to high (low), the P type dopant concentration of HD layer L1 exceeded (fell short of) the desired concentration HD of HD layer L1 as indicated by the line 304 (306).
FIG. 4 is a schematic representation of process gas source 30 of FIG. 1 illustrating the dilution of a dopant gas DG with a carrier gas CG in accordance with the prior art. As shown in FIG. 4, process gas source 30 included a mixer 50. Coupled to mixer 50 was a check valve 52 through which carrier gas CG, e.g. hydrogen, flowed. Also coupled to mixer 50 was a mass flow controller (MFC) 54 through which dopant gas DG flowed. In mixer 50, carrier gas CG and dopant gas DG mixed. The mixture of carrier gas CG and dopant gas DG was supplied as process gas C to a mass flow controller (MFC) 56 and to MFC 36. MFC 36 controlled and regulated the mass flow rate of the flow of process gas C to reactor 14 as discussed above. MFC 56 controlled and regulated the mass flow rate of the flow of process gas C to exhaust 23.
Observation of the dopant concentration in a layer formed using this apparatus reveals variations in the dopant concentration within the layer from batch to batch. While for conventional devices the variations are not significant, the variations are not acceptable for emerging process technologies that require a substantially constant doping concentration within a layer. Consequently, in addition to the problems with formation of transition layers, the present configurations do not produce the desired uniform doping level that is needed. Therefore, to achieve high volume quality production of thin layers, new apparatus and configurations will be required.
In accordance with the present invention, a gas flow controller system is located directly adjacent to a point of use that typically is a semiconductor processing reactor. This configuration eliminates the long prior art gas supply line that was between the gas manifold and the point of use. Consequently, this configuration eliminates the requirement to evacuate and/or purge the long prior art gas supply line through the point of use, which in turn results in a significantly faster reduction in the concentration of the process gas at the point of use, e.g., the semiconductor processing reactor. The significantly faster reduction in the process gas concentration significantly reduces or even eliminates the prior art transition layer. Thus, the gas flow controller system of this invention permits formation of abrupt transitions between layers.
In addition, the gas flow controller includes a novel configuration that permits stabilizing a second process gas flow simultaneously with supplying a first process gas flow to the semiconductor processing reactor. This reduces the process cycle time which in turn permits processing of more substrate batches in a given time period compared to the prior art processes.
The gas flow controller of this invention also provides for mixing predefined flows of a plurality of gases to create a substantially uniform concentration process gas flow. The substantially uniform concentration process gas flow is controlled so that a predefined portion is directed to the semiconductor processing reactor and a remainder is exhausted. Since the process gas flow from the mixing remains constant, a dopant concentration in the gas flow is precisely controlled. The precise control of the dopant concentration results in the formation of a doped layer on a substrate having a precise predefined concentration across the thickness of the layer in any particular batch and from batch to batch.
In one embodiment of the present invention, a gas flow control system for a semiconductor processing unit includes a first mass flow controller located at a first location and a support structure located at the semiconductor processing unit. The system further includes a gas manifold located at the support structure and a first gas manifold inlet valve located at the support structure and coupled between the gas manifold and the first mass flow controller. The gas manifold and the first gas manifold inlet valve are located at a second location separate and removed from the first location. The gas manifold is coupled through a process gas supply line to one or more injector ports of a reactor in which substrates, e.g., silicon wafers, are processed.
Of importance, the reactor is supported by the support structure, e.g., a table. Since the gas manifold and the gas manifold inlet valve are also located at the support structure as close as physically possible to the reactor, the length of the gas manifold and the process gas supply line is relatively short, e.g., is two feet or less, compared to the corresponding prior art gas supply line between the gas manifold and the one or more injector ports which was relatively long, e.g., ten feet or more. Since the relatively short length of the gas manifold and the process gas supply line holds a small amount of process gas, the process gas within the gas manifold and the process gas supply line is removed in a relatively short time, e.g., thirty seconds or less, after the flow of the process gas to the gas manifold is shut off. Advantageously, the relatively short time in accordance with the present invention allows realization of an abrupt transition between layers formed on a substrate.
Also in accordance with the present invention, a method of controlling gas flow to a reactor includes opening a first gas manifold inlet valve coupled between a first mass flow controller, e.g., a first regulator, and a gas manifold and regulating a mass flow rate of a flow of a first process gas through the first gas manifold inlet valve to the gas manifold with the first mass flow controller. The method further includes opening a gas manifold exhaust valve coupled between a second mass flow controller, e.g., a second regulator, and an exhaust and regulating a mass flow rate of a flow of a second process gas through the gas manifold exhaust valve to the exhaust with the second mass flow controller.
Of importance, the second process gas flows through the second mass flow controller thus stabilizing the mass flow rate of the flow of the second process gas through the second mass flow controller while the first process gas is flowing to the gas manifold and thus to the reactor coupled to the gas manifold.
This is in contrast to the prior art where all the process gases were sent to exhaust through a single common gas manifold and valve thus precluding the possibility of stabilizing a mass flow rate of a flow of process gas through any of the mass flow controllers while another process gas was being supplied to the reactor. Advantageously, stabilizing the mass flow rate of the flow of the second process gas through the second mass flow controller while the first process gas is being supplied to the reactor in accordance with the present invention improves cycle time and thus reduces the cost of processing substrates compared to the prior art.
Also in accordance with the present invention, a system for diluting a dopant gas with a carrier gas includes a mixer, a dopant gas source, e.g., a first gas source, coupled to an inlet port of the mixer and a carrier gas source, e.g., a second gas source, coupled to the inlet port of the mixer. The system further includes a first mass flow controller, e.g., a first regulator, coupled between the inlet port of the mixer and the dopant gas source and a second mass flow controller, e.g., a second regulator, coupled between the inlet port of the mixer and the carrier gas source. Coupled to an outlet port of the mixer are a third mass flow controller, e.g., a third regulator, and a check valve.
During use, the dopant gas is diluted with the carrier gas to result in a process gas having a desired dopant concentration. Since the mass flow rates of the flows of the dopant gas and the carrier gas to the mixer are controlled and regulated by the first and second mass flow controllers, respectively, and are constant, the dopant concentration of the process gas is likewise constant. Thus, regardless of what percentage of the flow of process gas is directed to exhaust through the check valve versus directed to the reactor through the third mass flow controller, the dopant concentration of the process gas is precisely determined and remains constant. This, in turn, results in the formation of a doped layer on a substrate having a precise and constant dopant concentration across the entire thickness of the doped layer in any particular batch as well as from batch to batch. For this reason, a system in accordance with the present invention is well suited to meet and exceed the stringent requirements of existing and emerging process technologies.
Also in accordance with the present invention, a method of diluting a dopant gas with a carrier gas includes setting a mass flow rate of a flow of the dopant gas to a mixer and setting a mass flow rate of a flow of the carrier gas to the mixer, where the dopant gas and the carrier gas mix in the mixer to form a process gas which flows out of the mixer. The method further includes setting a mass flow rate of a flow of the process gas to a reactor, where a difference between the flow of the process gas out of the mixer and the flow of the process gas to the reactor is excess process gas which is directed to exhaust.
These and other features and advantages of the present invention will be more readily apparent from the detailed description set forth below taken in conjunction with the accompanying drawings.